serial: xilinx_uartps: Use helper function hrtimer_update_function()

The field 'function' of struct hrtimer should not be changed directly, as
the write is lockless and a concurrent timer expiry might end up using the
wrong function pointer.

Switch to use hrtimer_update_function() which also performs runtime checks
that it is safe to modify the callback.

Signed-off-by: Nam Cao <namcao@linutronix.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/af7823518fb060c6c97105a2513cfc61adbdf38f.1738746927.git.namcao@linutronix.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>